Process for making a mat composed of aluminum silicate fibers and polytetrafluoroethylene



United States Patent William Happoldt, Jr., Claymout, Del., assignor toE. I. du Pont de Nemours and Company, Wilmington, Del., a corporation ofDelaware No Drawing. Application July 28, 1954 Serial No. 446,395

2 Claims. (Cl. 260-41) This invention relates to the preparation ofreinforced gasket stock composed of polytetrafiuoroethylene and mineralfibrous material.

It has been known heretofore that fibrous polytetrafiuoroethylene can beused as a packing or gasket material. This advantages ofpolytetrafluoroethylene in such applications include excellent corrosionresistance and resistance to harmful eifects of high temperatures.

An object of this invention is to provide gasket stock which containspolytetrafluoroethylene in combination with a mineral fibrous material.Another object is to provide a novel process for manufacturing gasketstock. Other objects of the invention will appear hereinafter.

The aforesaid objects are accomplished in accordance with thisinvention, by coagulating an aqueous dispersion ofpolytetrafluoroethylene, preferably in colloidal form, in the presenceof the mineral fibrous material, by addition of a coagulating agent Suchas aluminum sulfate.

The preferred mineral fibrous material to be used in the process of thisinvention is Fiberfrax, a fibrous form of aluminum silicate which isbelieved to be produced by blowing a falling stream of the moltenmaterial as it is formed from an electric furnace. It is composedchiefly of equal parts of silica and alumina, has a melting point ofabout 3000 F. and a fiber diameter of one to eight microns. The fiberlength is up to 3 inches but is mostly under 0.5 inch. The material ismanufactured by the Carborundum Company, Niagara Falls, New York.

The polytetrafiuoroethylene dispersions which are employed arepreferably concentrated to at least about solids, prior to introductioninto the dilute slurry of mineral fiber. The quantity of dispersingagent is highly critical, because if this quantity exceeds a maximumpermissable amount, the polytetrafluoroethylene will not exhaust uponthe mineral fibers. The critical amount of dispersing agent, based onthe weight of the polytetrafluoroethylene, is about 2% of Triton X-100(an alkyl aryl polyether alcohol non-ionic surface active agent, morespecifically a polyethylene glycol-p-octylphenol ether made by Rohm andHaas Company), or

a quantity of other dispersing agent having the same dispersing orwetting power. If the quantity of dispersing agent is too large, thedesired products are not obtained.

The coagulating agent can be any highly ionizable material, preferably asubstance which yields ions of a polyvalent metal, such as aluminum.Ordinary alum (sodium potassium aluminum sulfate), or A1 (SO in commonhydrated forms, or other similar salts are highly eifective. Thequantity which is required is not highly critical.

The process of the invention is preferably carried out in ordinarypaper-making equipment, although other types or apparatus fordisintegrating fibrous masses and admixing additives with fibrousmaterial can also be used. After the ingredients are mixed together, theresulting fibrous mass is removed, sheeted, and compressed, suitably bymeans of a press capable of delivering at least about 1000 lbs. per sq.inch of pressure to the gasket stock. The resulting sheet can be driedand sintered at elevated temperatures, suitably up to 400 C.

The invention is illustrated further by means of the following example:

Example.A 0.7% (by weight) suspension of Fiberfrax fibers in water isprepared by beating the Fiberfrax fibers sufficiently gently in water toavoid any appreciable or notable shortening of the fiber length. Thisrequires about five minutes. To the resulting suspension is added aquantity of colloidal dispersion of polytetrafluoroethylene (17% byweight) corresponding to 60 parts of polytetrafluoroethylene per 40parts of the mineral fiber. A 1% solution of aluminum sulfate in water(about 5% of the total volume) is next added, and the gentle agitationis continued. The pH of the mixture at this stage is about 5. A mat ofthe resulting fibers is next produced upon a wire screen, using theconventional paper-making technique. Drying the resulting mat isaccomplished first by applying a blotting paper, and next by means of adrum drier operating at 250 F. Finally the mat is compressed andsintered at 400 C. The cooling of the mat is performed under sufiicientpressure (1000 pounds per square inch) to keep the mat dense. Theresulting product is a highly efiective gasket material.

The quantity of dispersing agent, Triton X-lOO, hereinabove defined, inthe dispersion employed in the foregoing example was 1%, based on theweight of polytetrafluoroethylene. Repetition of the experiment with 5%of this agent failed to produce exhaustion of thepolytetrafluoroethylene.

I claim:

1. A process which comprises adding a highly ionizable coagulating agentto a dispersion of colloidal polytetrafiuoroethylene in water containingfibrous aluminum silicate, said dispersion being further characterizedin that it is obtained by adding concentrated aqueous colloidalpolytetrafiuoroethylene, having a polytetrafiuoroethylene content of 10to 17% by weight to a fibrous aluminum silicate aqueous dispersionhaving a fibrous aluminum silicate content of about 0.7% by weight, saiddispersion containing a controlled quantity of dispersing agentsulficient to prevent coagulation of the dispersion except upon additionof said highly ionizable material, the weight ratio ofpolytetrafluoroethylene: fibrous aluminum silicate being about 60:40,agitating the resulting mixture until the said polytetrafluoroethylenebecomes exhausted upon the said fibrous aluminum silicate, at a pH ofabout 5, thereafter forming a mat of the resulting fibers, drying andsintering the said mat, said controlled quantity of dispersing agentbeing not in excess of 2% by weight, based on the weight of saiddispersion, said dispersing agent being an alkyl aryl polyether alcohol.

2. Process of claim 1 wherein the said coagulating agent is aluminumsulfate.

References Cited in the file of this patent UNITED STATES PATENTS2,593,583 Lontz Apr. 22, 1952 2,732,357 Sprung Ian. 24, 1956 FOREIGNPATENTS 1,031,164 France Mar. 18, 1953

1. A PROCESS WHICH COMPRISES ADDING A HIGHLY IONIZABLE COAGULATING AGENTTO A DISPERSION OF COLLOIDAL POLYTETRAFLUOROETHYLENE IN WATER CONTAININGFIBROUS ALUMINUM SILICATE, SAID DISPERSION BEING FURTHER CHARACTERIZEDIN THAT IT IS OBTAINED BY ADDING CONCENTRATED AQUEOUS COLLOIDALPOLYTETRAFLUOROETHYLENE, HAVING A POLYTETRAFLUOROETHYLENE CONTENT OF 10TO 17% BY WEIGHT TO A FIBROUS ALUMINUM SILICATE AQUEOUS DISPERSIONHAVING A FIBROUS ALUMINUM SILICATE CONTENT OF ABOUT 0.7% BY WEIGHT, SAIDDISPERSION CONTAINING A CONTROLLED QUANTITY OF DISPERSING AGENTSUFFICIENT TO PREVENT COAGULATION OF THE DISPERSING EXCEPT UPON ADDITIONOF SAID HIGHLY IONIZABLE MATERIAL, THE WEIGHT RATIO OFPOLYTETRAFLUOROETHYLENE: FIBROUS ALUMINUM SILICATE BEING ABOUT 60:40,AGITATING THE RESULTING MIXTURE UNTIL THE SAID POLYTETRAFLUOROETHYLENEBECOMES EXHAUSTED UPON THE SAID FIBROUS ALUMINUM SILICATE, AT A PH OFABOUT 5, THEREAFTER FORMING A MAT OF THE RESULTING FIBERS, DRYING ANDSINTERING THE SAID MAT, SAID CONTROLLED QUANTITY OF DISPERSING AGENTBEING NOT IN EXCESS OF 2% BY WEIGHT, BASED ON THE WEIGHT OF SAIDDISPERSION, SAID DISPERSING AGENT BEING AN ALKYL ARYL POLYETHER ALCOHOL.